Ok, so sites that are updated constantly are more likely to get into Google's good graces than not. But how much work doe it require?

For example, today I rewrote 3 teasers and 3 section descriptions, as well as renamed those sections to something more keyword-friendly. Is that enough to keep Google happy? Or does Google require that you actually add new stuff to your site every day?

It depends entirely on your competition. Google likes to serve the fresher content but that is still only part of the criteria. If two sites are the same weight in key words and link backs then the site with the fresher content will win out. I have some sites with some relatively stale content that still rank on the front page because my competition's content is just as stale.

Oh, ok! Is that why you'll see a fansite for a t.v. series that was cancelled years ago still the #1 search result, even though the last entry for the site was like Otober of 2004?